1. Field of the Invention
The present invention relates generally to a technique of canceling memory effects in secondary batteries, particularly to a technique of canceling memory effects in secondary batteries used for applications in which the secondary batteries are charged and discharged repeatedly in a charged but not fully charged state and it is difficult to discharge the secondary batteries completely.
2. Related Background Art
Examples of the secondary battery include lead batteries, nickel-cadmium (Ni-Cd) batteries, nickel metal-hydride (Ni-MH) batteries, and lithium ion batteries. Such batteries have a characteristic allowing them to be connected to an external power source to be charged with a predetermined current supplied from the power source after consumption of their electric power. These batteries have been used for various equipment, making good use of such a characteristic. For instance, such a battery may be mounted on a vehicle to supply electric power to an ignition plug of its engine. Recently, such a battery also is used as a main power source for driving an electric motor in a pure electric vehicle (PEV) or a so-called hybrid electric vehicle (HEV) provided with an engine and an electric motor.
For instance, in the case of a secondary battery mounted on a HEV, the secondary battery may be charged and discharged repeatedly while the HEV is driven. In the HEV, when the output of its engine is higher than the required power for driving, a generator is driven with the surplus power to charge the secondary battery. On the contrary, when the output of the engine is lower than the required power, an electric motor is driven with the electric power of the secondary battery to output power sufficient to cover the shortage of the required power. In this case, the secondary battery is discharged. Such repeated charge and discharge are carried out according to the driving state of the vehicle, the charged state of the battery, and an operation of a driver.
In this manner, when an incomplete charge and discharge of the secondary battery are repeated, the actually usable capacity is reduced accordingly and a so-called memory effect occurs. Usually, the secondary battery with its capacity reduced temporarily due to this memory effect can recover its actual capacity by being discharged completely.
For instance, in the case of the Ni-MH battery, the memory effect can be cancelled when the battery is forced to be discharged until the voltage per cell drops to 1V.
In the secondary battery system to be loaded on a HEV, however, a number of cells are connected in series to form a battery assembly in order to obtain the required output for driving an electric motor, and a battery voltage, current, and temperature are monitored so that the charge and discharge are repeated with the state of charge (SOC) of the secondary battery being in a range of 30% to 75% of a fully charged state. In such a system, when there is complete discharge for the cancellation of the memory effect during driving of the vehicle, there is a possibility that a severe trouble such as stoppage of the vehicle on the road may be caused.
Hence, it has been necessary to stop the vehicle at a charging site or the like, to discharge the secondary battery completely, and then to charge the secondary battery for a predetermined time until the secondary battery is fully charged.
With the foregoing in mind, it is an object of the present invention to provide a charging/discharging control device and method in which the memory effect can be cancelled during driving of the vehicle.
In order to achieve the above-mentioned object, a first charging/discharging control device according to the present invention includes a secondary battery, a memory effect determinator, and a discharging controller. The secondary battery is formed of a battery assembly with a plurality of cells combined with one another. The memory effect determinator determines whether a discharging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of the secondary battery has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V1 at a lower-limit state of charge in a range of capacity actually used. The discharging controller discharges the secondary battery until the voltage per cell reaches a voltage V3 that is higher than 1.0 V and is lower than the voltage V1 at the lower-limit state of charge when the memory effect determinator determines that the discharging memory effect has occurred.
In order to achieve the above-mentioned object, a second charging/discharging control device according to the present invention includes a secondary battery, a memory effect determinator, and a discharging controller. The secondary battery is formed of a battery assembly with a plurality of cells combined with one another. The memory effect determinator determines whether a discharging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of the secondary battery has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V1 at a lower-limit state of charge in a range of capacity actually used. The discharging controller discharges the secondary battery for a predetermined amount from the lower-limit state of charge when the memory effect determinator determines that the discharging memory effect has occurred.
Preferably, the first and second charging/discharging control devices further include a charging controller for charging the secondary battery until the voltage per cell reaches a voltage V4 that is higher than a voltage V2 at an upper-limit state of charge in the range of capacity actually used after completion of the discharging of the secondary battery by the discharging controller based on the determination by the memory effect determinator that the discharging memory effect has occurred.
It is preferable that discharging and charging of the secondary battery are repeated in this order at least once, wherein the discharging is carried out by the discharging controller until the voltage per cell reaches the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, from the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge, and the charging is carried out by the charging controller until the voltage per cell reaches the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge, from the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge.
Preferably, the first charging/discharging control device further includes a charging level corrector for correcting a charging level at the lower-limit state of charge based on a discharging amount when discharging is carried out until the voltage per cell reaches the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, from the voltage V1 by the discharging controller.
Preferably, the second charging/discharging control device further includes a charging level corrector for correcting a charging level at the lower-limit state of charge based on the voltage per cell after the secondary battery is discharged for the predetermined amount from the lower-limit state of charge by the discharging controller.
In order to achieve the above-mentioned object, a third charging/discharging control device according to the present invention includes a secondary battery, a memory effect determinator, and a charging controller. The secondary battery is formed of a battery assembly with a plurality of cells combined with one another. The memory effect determinator determines whether a charging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of the secondary battery has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V2 at an upper-limit state of charge in a range of capacity actually used. The charging controller charges the secondary battery until the voltage per cell reaches a voltage V4 that is higher than the voltage V2 at the upper-limit state of charge when the memory effect determinator determines that the charging memory effect has occurred.
In order to achieve the above-mentioned object, a fourth charging/discharging control device according to the present invention includes a secondary battery, a memory effect determinator, and a charging controller. The secondary battery is formed of a battery assembly with a plurality of cells combined with one another. The memory effect determinator determines whether a charging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of the secondary battery has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V2 at an upper-limit state of charge in a range of capacity actually used. The charging controller charges the secondary battery for a predetermined amount from the upper-limit state of charge when the memory effect determinator determines that the charging memory effect has occurred.
Preferably, the third and fourth charging/discharging control devices further include a discharging controller for discharging the secondary battery until the voltage per cell reaches a voltage V3 that is lower than a voltage V1 at a lower-limit state of charge in the range of capacity actually used after completion of the charging of the secondary battery by the charging controller based on the determination by the memory effect determinator that the charging memory effect has occurred.
It is preferable that charging and discharging of the secondary battery are repeated in this order at least once, wherein the charging is carried out by the charging controller until the voltage per cell reaches the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge, from the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, and the discharging is carried out by the discharging controller until the voltage per cell reaches the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, from the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge.
In order to achieve the above-mentioned object, a first charging/discharging control method according to the present invention includes: determining whether a discharging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of a secondary battery formed of a battery assembly with a plurality of cells combined with one another has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V1 at a lower-limit state of charge in a range of capacity actually used; and discharging the secondary battery until the voltage per cell reaches a voltage V3 that is higher than 1.0 V and is lower than the voltage V1 at the lower-limit state of charge when it is determined that the discharging memory effect has occurred.
In order to achieve the above-mentioned object, a second charging/discharging control method according to the present invention includes determining whether a discharging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of a secondary battery formed of a battery assembly with a plurality of cells combined with one another has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V1 at a lower-limit state of charge in a range of capacity actually used; and discharging the secondary battery for a predetermined amount from the lower-limit state of charge when it is determined that the discharging memory effect has occurred.
Preferably, the first and second charging/discharging control methods further include charging the secondary battery until the voltage per cell reaches a voltage V4 that is higher than a voltage V2 at an upper-limit state of charge in the range of capacity actually used after completion of the discharging of the secondary battery based on the determination that the discharging memory effect has occurred.
It also is preferable that discharging and charging of the secondary battery are repeated in this order at least once, wherein the discharging is carried out until the voltage per cell reaches the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, from the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge, and the charging is carried out until the voltage per cell reaches the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge, from the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge.
Preferably, the first charging/discharging control method further includes correcting a charging level at the lower-limit state of charge based on a discharging amount when the discharging is carried out until the voltage per cell reaches the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, from the voltage V1.
Preferably, the second charging/discharging control method further includes correcting a charging level at the lower-limit state of charge based on the voltage per cell after the secondary battery is discharged for the predetermined amount from the lower-limit state of charge.
In order to achieve the above-mentioned object, a third charging/discharging control method according to the present invention includes determining whether a charging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of a secondary battery formed of a battery assembly with a plurality of cells combined with one another has reached a predetermined time; whether a charging/discharging amount in the second battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V2 at an upper-limit state of charge in a range of capacity actually used; and charging the secondary battery until the voltage per cell reaches a voltage V4 that is higher than the voltage V2 at the upper-limit state of charge when it is determined that the charging memory effect has occurred.
In order to achieve the above-mentioned object, a fourth charging/discharging control method according to the present invention includes: determining whether a charging memory effect has occurred, based on at least one of the following determinations: whether a time of charging/discharging of a secondary battery formed of a battery assembly with a plurality of cells combined with one another has reached a predetermined time; whether a charging/discharging amount in the secondary battery has reached a predetermined amount; and whether a voltage per cell has reached a voltage V2 at an upper-limit state of charge in a range of capacity actually used; and charging the secondary battery for a predetermined amount from the upper-limit state of charge when it is determined that the charging memory effect has occurred.
Preferably, the third and fourth charging/discharging control methods further include discharging the secondary battery until the voltage per cell reaches a voltage V3 that is lower than a voltage V1 at a lower-limit state of charge in the range of capacity actually used after completion of the charging of the secondary battery based on the determination that the charging memory effect has occurred.
It also is preferable that charging and discharging of the secondary battery are repeated in this order at least once, wherein the charging is carried out until the voltage per cell reaches the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge, from the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, and the discharging is carried out until the voltage per cell reaches the voltage V3, which is lower than the voltage V1 at the lower-limit state of charge, from the voltage V4, which is higher than the voltage V2 at the upper-limit state of charge.
According to the configurations and methods described above, the memory effect caused during discharging is detected based on the charging/discharging time, charging/discharging amount, and voltage drop to the voltage corresponding to the lower-limit state of charge of the secondary battery during driving of the vehicle. Then, the battery is discharged to reach a lower state of charge than a lower-limit state of charge in the range of capacity actually used and thus the discharging memory effect in the range of capacity actually used is cancelled. In addition, the charging level at the lower-limit state of charge is corrected, so that the state of charge is controlled accurately. Afterward, the secondary battery is charged to reach a higher state of charge than the upper-limit state of charge, so that the charging memory effect also can be cancelled.
During driving of the vehicle, the memory effect caused during charging is detected based on the charging/discharging time, charging/discharging amount, and voltage rise to the voltage corresponding to the upper-limit state of charge in the range of capacity actually used of the secondary battery. The battery is charged to reach a higher state of charge than the upper-limit state of charge, so that the charging memory effect in the range of capacity actually used is cancelled. Afterward, the battery is discharged to reach a lower state of charge than the lower-limit state of charge, so that the discharging memory effect also can be cancelled.